1. Field of the Invention
The present invention relates to a complex oxide ceramic sintered body, which is particularly used as a material for separators in solid-electrolyte fuel cells, and to a solid-electrolyte fuel cell using the same.
2. Description of the Related Art
Separators in solid-electrolyte fuel cells must be highly conductive, dense and stable in both oxidizing and reducing atmospheres. The separators must also have a thermal expansion coefficient close to that of yttria-stabilized-zirconia (hereinafter referred to as "YSZ") which is generally used as a solid electrolyte material.
Although lanthanum chromite (hereinafter referred to as "LaCrO.sub.3 ") is known to be stable in a high-temperature oxidizing atmosphere and in a low oxygen partial pressure atmosphere, LaCrO.sub.3 has poor sintering properties and insufficient denseness. Therefore, when LaCrO.sub.3 is used for separators of solid-electrolyte fuel cells, the fuel gas and air are not completely separated from each other, which is not satisfactory. Additionally, since LaCrO.sub.3 has a significantly low thermal expansion coefficient in comparison with the thermal expansion coefficient (10.0 to 11.0.times.10.sup.-6 K.sup.-1) of YSZ used as a solid electrolyte, bonded sections are easily separated because of the difference in volumetric change during heating and cooling. Moreover, LaCrO.sub.3 does not have satisfactory conductivity.
Ceramics in which the site of La in LaCrO.sub.3 is partially substituted by an alkali metal such as Ca or Sr, and in which the site of Cr is partially substituted by a transition metal such as Co, Ni, Cu, Zn, Fe or Mn, are disclosed as perovskite-type oxides which are highly conductive and are easily sintered (refer to Japanese Unexamined Patent Publications Nos. 3-65517, 4-214069, 4-331764, and 6-16471). However, ceramics sintered bodies having these compositions expand in a low oxygen partial pressure atmosphere and are unstable because of a decrease in strength, and thus warping may occur in cells or failure may be caused by small stresses if used under operating conditions in solid-electrolyte fuel cells.
The stability of the ceramic sintered bodies in a low oxygen partial pressure atmosphere is improved by controlling the amount of an alkali metal (for example, Ca or Sr) which partially substitutes the La site and the amount of metallic elements (at least one of Co and Ni, and at least one of Al and Mg) which partially substitute the Cr site, as disclosed in Japanese Unexamined Patent Publication No. 8-59341. However, this ceramic requires a sintering temperature of 1,750.degree. C. or more, and the phase transition temperature thereof has not been examined. Therefore, when the ceramic sintered body is used as a separator in a solid-electrolyte fuel cell, contact failure with the solid-electrolyte material or other components, deformation or the like may occur because of volumetric change due to the phase transitions during repeated heating and cooling.
The phase transition temperature of the ceramic sintered body is decreased to 25.degree. C. or less by partially substituting the La site by an alkali metal (for example, Ca or Sr) and by partially substituting the Cr site by Al only, as disclosed in Japanese Unexamined Patent Publication No. 9-196098. With respect to this ceramic, however, a phase transition temperature of 25.degree. C. is not sufficient in consideration of use and transportation in winter and in cold areas, and a much lower phase transition temperature is desirable. Additionally, it is believed that the thermal expansion coefficient of the ceramic sintered body is not brought close to the thermal expansion coefficient of YSZ used as a solid electrolyte, and thus bonded sections may separate due to differences in volumetric changes during heating and cooling. Moreover, stability in an atmosphere of low oxygen partial pressure has not been fully examined.